dt-bindings:iio:adc:nuvoton,npcm750-adc yaml conversion

Renamed the file to reflect the only compatible.
Added #io-channel-cells to make it easier to support consumers of the
ADC channels this device provides.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Tomer Maimon <tmaimon77@gmail.com>
Link: https://lore.kernel.org/r/20200909175946.395313-9-jic23@kernel.org